The long-term goal of this proposal is to understand the structures and activation of mechanisms of rod and cone pigments. Several mutations in rhodopsin, the rod pigment, have been shown to result in gain or loss of function and ultimately lead to retinal degeneration. However, similar degenerative diseases have not been reported from cone pigment mutations. The dearth of information on cone pigments or inherent differences of cone pigment structure and/or activation are two possible reasons. More data on the basic properties of bone pigments will shed light on eith4r scenario. This proposal is directed at determining the important protein-ligand interactions, particularly around the 9-methyl group of 11-cis retinal, in the activation and inactivation of rod and cone pigments. There are three aims to this proposal: (1) establishment of a light-independent active form of a cone pigment consistent with a "steric trigger" mechanism involving amino acids different from those previously published by others; (2) test of the "salt bridge" model of activation in the cone pigments; (3) determination of the role of the extracellular domains on the stability and activation of visual pigments. Spectroscopic and transducin activity measurements of expressed salamander visual pigments will be used. These results will also be important in the understanding of the activation mechanisms of the larger field of G protein-coupled receptors.